Introduction
Systems that protect valuable content require effective security. For content distributed in physical form, such as film being transported to movie theaters, physical security measures can be sufficient. Unfortunately, traditional physical security techniques are slow, expensive, cumbersome, and cannot be used with non-physical content distribution models. As a result, content providers rely on cryptographic hardware to ensure that only authorized users can access their data.
To prevent misuse of decryption keys, cryptographic hardware used to manage content decryption keys must be tamper-resistant. Building effective tamper resistant hardware has proven extremely difficult, especially for systems that are the subject of determined attacks, because they are large or protect high-value content. As a result, many systems (including most satellite television systems) use replaceable security devices, such as smartcards, so that security can be re-established after an attack without replacing the entire playback system. Nevertheless, smartcards used for prepaid telephone, pay-TV, and transit applications are broken regularly. For example, prepaid telephone cards used in Germany were attacked in 1998 with estimated losses of US$38 million ("Pirates Cash in on Weak Chips," Wired News, May 22, 1998). Similarly, access cards and systems for cable and prepaid satellite television services are regularly "hacked," necessitating repeated costly card replacements.
Smartcards must resist a variety of attacks against cryptographic algorithms, protocols, software, and chip hardware. Unfortunately, designing a smartcard that implements sophisticated protocols yet contains no security flaws has proven to be a very difficult task, since unexpected problems or errors in any portion of the design can render the entire card insecure. Cost considerations also favor attackers, since smartcards typically cost between $1 and $15, yet may be trusted to protect services or information worth thousands of dollars.
A smartcard system will only be attacked seriously if it is in the attacker's interest to break it. With smartcard designs of the background art, once attackers develop a means to compromise one card, the incremental cost to break a large number of cards is usually very small. As a result, smartcard security efforts typically focus on preventing the initial attack by making the card more difficult to break. For example, vendors try to increase the cost of reverse-engineering the device or imaging the card's ROM. Such techniques are helpful because they increase the cost required to break the system the first time, but for very large systems they are ineffective because attackers will devote enough effort to attacks that they will eventually succeed.